1. Technical Field
This invention is concerned with a method of quenching articles in a fluidized bed. It is especially useful for hardening particular metal alloys which exhibit reduced hardenability due to their alloy composition.
2. Background Art
Quenching of an article to affect the physical structural phases of the article is well known in the art. Quenching is generally understood to be a sudden change in temperature of the article. "Up-quenching" is a rapid increase in temperature, whereas "down-quenching" is a rapid decrease in temperature.
Quenching can be achieved in a number of different manners whereby a means is provided for rapid heat transfer to or from the article being quenched. If the article is quenched too rapidly, stresses may build up in the article causing undesired structural defects. Conversely, slow quenching can result sin the formation of several different physical/morphological structures within the article being quenched; typically the surface physical structure differs from the internal physical structure of the article, particularly for thick-wall structures. For many applications it is desired to maintain a constant physical structure throughout the article to avoid stress concentration.
Quenching of metals and alloys thereof for purposes of obtaining desired physical crystalline structures within the metal/alloy requires careful control over the quench rate. Fluidized beds have been used successfully to provide uniform, controlled heat transfer mediums for quenching purposes. The present applicants previously disclosed a process for quenching articles in a fluidized bed in U.S. patent application, Ser. No. 913,320, filed Sect. 30, 1986, which is hereby incorporated by reference into the present application. The disclosed process is shown to be particularly useful for quenching steel alloys to provide a particular crystalline structure, avoiding the formation of undesirable softer phases within the article. However, there are certain steels, particularly those comprising low overall carbon contents, below about 0.45 weight percent carbon, and those comprising low alloy metal contents combined with a carbon content of less than about 0.3 weight percent, which can be better quenched using the improvement over the method of U.S. Ser. No. 913,320 which is provided by the present invention. In addition, quenching throughput rate can be increased in general by use of the present invention, which provides an increased heat transfer coefficient between the article being quenched and the fluidized bed heat transfer medium.
Other known related art described fluidized bed characteristics and/or various methods used in the quenching of articles or workpieces. Such art includes U.S. Pat. No. 4,612,065 to Kuhn; U.S. Pat. No. 4,300,936 to Quilleuere et al.; U.S. Pat. No. 4,372,774 to Cross et al.; German Pat. DE 3429707 to Schwing et al.; Japanese Patent Application No. 81-199840; and Japanese Patent Application No. 71027934. R. Gupta and A. S. Mujumdar describe the aerodynamics of a vibrated fluid bed in the Canadian Journal of Chemical Engineering, Vol. 58, pp. 332-338, June 1980. Additional information regarding vibrated fluidized beds is provided by A. D. Tamarin, I. I. Kal'tman and L. A. Vasil'ev in an article titled "Vibrofluidized Bed for Quenching", translated from Metallovedenie & Termicheskaya Obrabotka Metallov, No. 3, pp. 10-11, March 1968.
The latter article by Tamarin et al. discusses the use of fluidized bed vibration to improve the heat exchange rate during quenching of a copper ball. The copper ball was heated to about 850.degree. C. and then rapidly immersed into a vibrofluidized bed of particles. Corundum (average particle size 45.9.mu.) and sand (average particle size 201.6.mu.) were placed in cylindrical tanks 140mm in diameter which were subjected to vertical vibration at a frequency of 16 cps at an amplitude of 3-6mm. The authors claim to have obtained high heat transfer rates, and from the curves published in the article, the apparent heat transfer rate is about 300 BTU/hr. ft.sup.2 .degree. F. (0.170 joules/sec. cm.sup.2 .degree. C.) at about 600.degree. C. (1,110.degree. F.). The high heat transfer rate reported may in part be due to the very high thermal conductivity of copper. The article is silent regarding the gas used to fluidize the bed, which is, then, presumed to be air or nitrogen.
Applicants' laboratory experiments indicate a heat transfer rate of about 150-180 BTU/hr ft .sup.2 .degree. F. should be obtained for a nickel sphere down quenched under the conditions described by A. I. Tamarin et al. This presumes nitrogen or air is used as the fluidizing gas in the fluidized bed.
All of the above related art describes fluid bed characteristics and methods of operating fluid beds directed toward generation of more rapid heat transfer between the article being treated and the fluidized bed. However, the technology disclosed does not provide sufficiently high heat transfer rates to enable quenching of carbon steels comprising low overall carbon content (for example, steels such as 1045, 1050, 1140 and 1524) at a sufficiently rapid rate to provide a desired crystalline structure while avoiding the formation of softer phases within the article. Examples of alloy steels for which similar problems occur during quenching include 4130, 4120 and 5130.
It is desired, then, to provide a combination of fluidized bed parameters which generates an improved heat transfer rate sufficient to enable quenching of low carbon and low alloy content steels without the formation of softer phases within the article. The improved heat transfer rate is, of course, an advantage in the quenching of metals other than steel, such as heat-treatable aluminum alloys, and in the quenching of nonmetallic materials.
The method of the present invention discloses a combination of fluidized bed parameters which provides an unexpected improvement in the heat transfer rate between the articles being treated and the fluidized bed. Although some of the parameters are discussed individually in known related art, it is a particular combination of parameters not previously disclosed, including one parameter not previously known to provide a significant advantage. which generates the present unexpected improvement.